Step linestyle

Looked at this because of http://stackoverflow.com/questions/15188005/linestyle-in-matplotlib-step-function/15191183#15191183

The `step` function is just a light wrapper of `plot`.  Added a step to append the user specified `linestyle` onto the argument passed to `plot` instead of silently ignoring it.

Includes a unit test (there is currently no other test-coverage of `step`).

Based this against master because I am not sure if this is really a bug, or if I am changing an intentional interface.